There is known a system to turn on one of relays connected in parallel, in a switching circuit for switching power between a power supply and a load, and thereafter to turn on the other relay after a delay time for stabilizing a surge current (Patent Document 1).
Recently, investigation is under way for the use of a parallel combination of a plurality of secondary batteries as power supply for supplying power to electric load (hereinafter referred simply as load) for facilities and homes and for electric vehicles. In the power supply of this type, a relay is connected in series with each of the secondary batteries and the secondary batteries are connected in parallel through the relays because there is a need for connecting the secondary batteries in such a disconnectable manner to electrically disconnect the secondary batteries individually for maintenance or for sensing the state of each secondary battery (such as an open circuit voltage).
However, in the power supply including the secondary batteries connected in parallel through the relays, when a plurality of the relays connected with the secondary batteries are turned on simultaneously, a current is generated at the output of the power supply by overlapping or superposition of surge currents generated in the relays, and the output current of the power supply might be increased excessively. To solve this problem, one conceivable system may employ two relays connected in parallel at the output of each secondary battery and turn on the relays by turning on one of the relays first and then turning on the other after a delay time to stabilize the surge current, as in the above-mentioned earlier technology. However, two relays are needed for each secondary battery and the cost is increased.